In recent years, the demand for compound semiconductors, especially Group III-V compounds (e.g. GaAs), has been growing because of their superior performance characteristics compared to the conventional silicon semiconductors. For the production of such compound semiconductors, there are known, among others, (a) the so-called molecular beam epitaxy (MBE) process which comprises causing atoms, required for a compound to be epitaxially grown, to evaporate from a solid material using a heat gun and causing them to collide, in the molecular beam form, against a substrate in an ultrahigh vacuum to thereby cause growth of a film of said material on said substrate, and (b) the so-called metal organic chemical vapor deposition (MOCVD) process which comprises introducing the vapor of a methyl-metal or ethyl-metal compound into a reaction chamber at atmospheric pressure or under reduced pressure by means of a carrier gas such as H.sub.2, allowing said vapor to mix with a Group V metal hydride and allowing the reaction therebetween to take place on a heated substrate for crystal growth.
However, the MBE process has a problem in that it is not suited for large-scale production, hence can hardly meet the needs of the market. The MOCVD process also has a problem in that the reactant gases are expensive and because of the growth mechanism, the efficiency of utilization of reactant gases is low, although the process is higher in production capacity than the above-mentioned MBE process. It is therefore difficult to use the MOCVD process for the production of semiconductors for use in fields other than some special fields where high costs do not matter. Furthermore, since a large quantity of unreacted gas, which is toxic, is produced because the efficiency of reactant gas utilization is low, as mentioned above, and since a carrier gas is used for gasifying and carrying the Group III compound, which has a low vapor pressure, and this constitutes an additional waste gas portion, a toxic waste gas is discharged in large quantities, and this fact leads to waste gas disposal problems. An apparatus so far in use for such a MOCVD process can be schematically illustrated by FIG. 11. Thus, a substrate 3 is mounted on a heater 2 disposed in a vacuum chamber 1, and a gaseous compound mixture for semiconductor deposition is fed toward the substrate 3, in the direction indicated by the arrows A, from a nozzle 4 disposed in the upper part of the vacuum chamber 1. In such an apparatus, the vacuum chamber 1, which has a large capacity, is filled with a gaseous compound (reactant gas) mixture for each treatment run and after treatment, the gaseous compound mixture is discharged as a waste gas. The waste gas contains unreacted gases, which have not been involved in semiconductor deposition, in large quantities and, accordingly, the reactant gas utilization efficiency is low. In the above-mentioned apparatus, the substrate 3 is placed on the heater 2 and heated from the under surface. Thermal convection thus takes place over the substrate, as shown by the arrows B, and heat is dissipated from the substrate 3 heated by the heater 2 to the vicinity of the upper surface of the substrate 3, as shown by the arrows C. As a result, the flow of the gaseous compound mixture fed from the nozzle 4 is disturbed by the forcing-up effect of the above-mentioned thermal convection current (arrows B) and heat dissipated (arrows C), so that uniform film growth on the upper surface of the substrate 3 cannot take place any longer. Therefore, it is a serious disadvantage of the apparatus mentioned above that the semiconductor films produced (semiconductor layers) do not have a smoothly finished surface. This disadvantage is further aggravated by the fact that GaAs particle flags formed by contact and reaction, in the gaseous atmosphere, of the reactants that have failed to arrive at the surface of the substrate 3 due to the abovementioned thermal convection (arrows B) float in the gaseous atmosphere and deposit on the semiconductor film at random. Furthermore, in the above-mentioned apparatus, any change in the type of feed gas is effected by operation of a plurality of valves 5, 6 and 7 for respective gas feeding pipes connected to the above-mentioned nozzle 4. For formation of a plurality of different types semiconductor layers, frequent change-over operation of the valves 5, 6 and 7 is required. This disadvantageously means a low operation efficiency. It is a further problem that the compound or compounds used in the previous series of runs remain in the nozzle 4 and behave as impurities which make it difficult to obtain semiconductors of good quality.
U.S. patent application Ser. No. 941,005 discloses an apparatus for producing semiconductors in a vacuum chemical epitaxy to solve the above-mentioned problems. A substrate is placed in a vacuum chamber. The substrate is heated from its topside. Mixing chambers are disposed in a multistage manner below the substrate. A plurality of reactant gases are supplied toward the substrate from the mixing chambers. However, the above-mentioned apparatus is basically for a batch system production process and unsuitable for a continuous operation. In a specification of the U.S. patent application, it is said that continuous operation is possible but an exact structure for the continuous operation is not disclosed at all.
Accordingly, it is an object of the invention to provide an apparatus for producing good-quality semiconductors having a smooth semiconductor layer surface efficiently by combining the advantages of the MBE process and those of the MOCVD.